This grant will support the scientific program of the Third International Congress of Twin Studies which will be sponsored by the International Society for Twin Studies in Jerusalem, Israel, in June, 1979. Symposia, workshops, meetings for contributed and invited papers, and a poster session are planned. Topics to be discussed include the biological, epidemiological, psychological, and genetic aspects of the twinning phenomenon as well as methodological and statistical aspects of the use of twins for biomedical research. Examples of applied research in the areas of birth defects, behavior genetics, pharmacogenetics, cardiovascular disease, cancer genetics, and obstetrical care will be presented and the proceedings of the Congress will be published to make them accessible to the general biomedical community. Twin studies have always played a major role in the genetic analysis of complex human traits. As increasingly sophisticated methodologies are developed and new phenotypes are examined, twin studies promise to assume an even greater importance during the next two decades as an efficient means of resolving genetic, environmental and maternal effects in man and for testing specific therapeutic hypotheses. The International Society for Twin Studies, through its publications and Congresses, can facilitate the realization of these potentialities.